An optical disk is formed as a disk-shaped substrate typically having one or more magnetic layers thereon. Information is stored on the disk in spiral or concentric tracks. Information is recorded on the disk and information is reproduced from the disk using a light beam that is focused on a desired track as the disk is rotated. The light beam is generated in an optical head and is directed at the disk surface in a perpendicular direction. The optical head does not contact the disk surface. It has been found necessary to provide a servo system for focus control and tracking control in order to record and reproduce correct information. The servo system is required to provide stable operation independent of variations in the signal processing system, the optical system, the disk driving system and individual differences among optical disk media.
By way of example, an optical disk may have a track pitch, or spacing, between adjacent tracks of 1.6 micrometers. However, the disk eccentricity may be up to 100 micrometers. Thus, a tracking servo is required to maintain the light beam on the desired track during recording and reproduction. The spacing between the objective lens of the optical system and the disk surface may be on the order of 1.5 mm, whereas the focal depth of the optical system may be on the order of one micrometer. Since the disk is not perfectly flat and may be tilted relative to the axis of rotation, a focus servo is required to maintain the light beam focused on the disk surface.
Various techniques are known for optical detection of focus errors and tracking errors. In the so called "knife edge" method of focus detection, a sharp edge is located at the focal point of the condenser lens in the optical system. A pair of photodetectors senses light reflected from the disk surface. When the beam is focused on the disk surface, the photodetectors provide equal outputs to a differential amplifier, and the output of the differential amplifier is zero. When the light beam is not focused on the disk surface, part of the reflected light is blocked by the knife edge and the differential amplifier provides a nonzero output voltage. The polarity of the output voltage indicates the direction of the focus error. Since the dynamic range of the focus sensor is small, it is customary to use a focus search to bring the focus servo into the range of the focus sensor.
The tracking sensor also utilizes a pair of photodetectors which receive the reflected beam from the disk surface. The outputs of the tracking photodetectors are connected to a differential amplifier. On the disk surface, each data track is typically centered between a pair of guide grooves. The guide grooves diffract the light beam. When the light beam is centered on the data track, the guide grooves on opposite sides of the data track diffract the light beam equally, and each photosensor receives the same signal. As the light beam deviates from the track center and moves closer to one of the grooves, the diffracted beam changes and the output of the differential amplifier increases. The polarity of the differential amplifier output represents the direction of the deviation from the track center. The tracking signal as the beam moves from an outer edge of the track to an inner edge of the track is a sinusoidal function of radial position and crosses the zero level at the center of the data track.
The servo systems utilized in an optical disk drive typically have offsets. Offsets are errors that occur even through the signals provided to the servo system indicate no errors. Offsets may originate for example from optical misalignment. In the prior art, it has been customary to adjust the optical disk drive in the factory to remove focus and tracking offsets. However, in spite of careful initial adjustments, additional offsets may be caused by environmental factors, shock and vibration, differences between media, aging and the like. When such offsets occur, accurate tracking and focus control cannot be achieved.
During recording of data on the optical disk, it is important to maintain the recording light beam on a predetermined track. If the light beam jumps to an adjacent track, data recorded on that track is likely to be destroyed. To insure that the light beam follows the desired track, the tracking signal discussed above is compared with a servo deviation threshold level. If the threshold level is exceeded, recording is interrupted. In the prior art, a fixed threshold level has been utilized. If the tracking signal varies in amplitude as a result of groove variations, disk tilt, focus errors and the like, a larger or smaller amplitude tracking signal is compared with a fixed threshold level. When the tracking signal is smaller than its normal amplitude, a larger deviation from the track center is required before the threshold level is exceeded. For very small amplitude tracking signals, the threshold level may not be reached for any deviation of the light beam from the desired track center. When the tracking signal is larger than its normal amplitude, the servo deviation threshold may be exceeded and recording may be interrupted, even though the light beam remains sufficiently aligned with the desired track.
It is desirable to provide an optical disk drive having a servo system which operates in a stable and accurate manner regardless of tracking offsets, focus offsets, sensor amplitude variations and the like. It is a general object of the present invention to provide improved optical disk drives.